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why is the color change of phenolphthalein colorless to pink, and not the opposite? isnt it supposed to be neutral at the end point..

(edited 7 years ago)

Original post by pondsteps

why is the color change of phenolphthalein colorless to pink, and not the opposite? isnt it supposed to be neutral at the end point..

Because it turns red at pH 9.7 ish, and adding one drop of NaOH to the acid just before neutralisation takes the pH to pH 10

Reply 2

alevels2k17

Original post by charco

Because it turns red at pH 9.7 ish, and adding one drop of NaOH to the acid just before neutralisation takes the pH to pH 10

but for methyl orange for eg. the end point is always orange, it doesnt take the color of the excess and change to yellow.. why does phenolphthalein take the color of the excess

Reply 3

PlayerBB

Original post by pondsteps

but for methyl orange for eg. the end point is always orange, it doesnt take the color of the excess and change to yellow.. why does phenolphthalein take the color of the excess

We added an alkaline (NaOH) to an acid solution, the final pH would be higher than before, phenolphthalein is red/pink in alkaline conditions and colourless in acid conditions, therefore since the pH is higher than before, the colour change would be from colourless to pink/red

same goes for methyl orange, it is orange/yellow in alkaline conditions and red in acidic conditions

Reply 4

Pigster

If you add acid to alkali you get one colour change; if you add alkali to acid, you get the reverse colour change.

We normally add acids to alkalis - it is usually easier to rinse acids out of burettes than it is alkalis.

Reply 5

alevels2k17

Original post by PlayerBB

so is the end point the neutral color or the excess of the reactant added(the titre)?

Reply 6

PlayerBB

Original post by pondsteps

so is the end point the neutral color or the excess of the reactant added(the titre)?

The excess of titre added

Posted from TSR Mobile

Reply 7

alevels2k17

Original post by PlayerBB

The excess of titre added

Posted from TSR Mobile

then why is methyl orange always orange at the end point of the titration not red :frown:

Reply 8

PlayerBB

Original post by pondsteps

then why is methyl orange always orange at the end point of the titration not red :frown:

No the colour change depend on the titre, on the reactant in the flask and on the pH change!!

For methyl orange, it turns orange when an alkaline solution is added (titre), the end point is when the colour change occurs from red to orange, it occurs because if there was an acid the the flask and an alkaline was added from beurette then at the end point, all of the acid would have been neutralised and excess alkali would be found in the flask which causes the colour change to occur

Reply 9

alevels2k17

Original post by PlayerBB

sooo now the end point is yellow?? not orange.. bcz there's excess alkali

Reply 10

PlayerBB

Original post by pondsteps

sooo now the end point is yellow?? not orange.. bcz there's excess alkali

You can say it's orange or yellow, it doesn't matter, the colour change occur from red to orange/yellow

I suggest revising these indicators stuff from the book, I know it's misleading so maybe the book will have a better shot than I am